The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press London--and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellent ink-receptive areas on a water-receptive ink-repellent background.
The DTR-image can be formed in the image receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
Two types of the mono-sheet DTR offset printing plate exist. According to a first type disclosed in e.g. U.S. Pat. No. 4,722,535 and GB-1,241,661 a support is provided in the order given with a silver halide emulsion layer and a layer containing physical development nuclei serving as the image-receiving layer. After information-wise exposure and development by means of an alkaline processing solution in the presence of a developing agent and a silver halide solvent the imaged element is used as a printing plate without the removal of the emulsion layer.
According to a second type of mono-sheet DTR offset printing plate a hydrophilic surface of a support is provided in the order given with a layer of physical development nuclei and a silver halide emulsion layer. After information-wise exposure and development by means of an alkaline processing solution in the presence of a developing agent and a silver halide solvent the imaged element is treated to remove the emulsion layer so that a support carrying a silver image is left which is used as a printing plate. Such type of lithographic printing plate is disclosed e.g. in U.S. Pat. No. 3,511,656.
From the above it will be clear that lithographic printing is only capable of reproducing two tone values because the areas will accept ink or not. Thus lithographic printing is a so called binary process. In order to reproduce originals having continuously changing tone values by such process halftone screening techniques are applied.
In a commonly used halftone screening technique, the continuously changing tone values of the original are modulated with periodically changing tone values of a superimposed two-dimensional screen. The modulated tone values are then subject to a thresholding process wherein tone values above the threshold value will be reproduced and those below will not be reproduced. The process of tone-value modulation and thresholding results in a two-dimensional arrangement of equally spaced "screen dots" whose dimensions are proportional to the tone value of the original at that particular location. The number of screen dots per unit distance determines the screen frequency or screen ruling. This screening technique wherein the screen frequency is constant and inversely proportional to the halftone cell size, is referred to as amplitude-modulation screening or autotypical screening. This technique can be implemented photo-mechanically or electronically.
It will further be clear that in order to reproduce a color image using lithographic printing it will be required to separate the image in three or more part-images corresponding to primary colors that when printed over each other yield the desired color at any place within the image. Each of these color separations has to be screened as described above.
It is well known that the above described procedure of screening results in certain artifacts on a copy obtained in lithographic printing. Such artifacts are e.g. enlarging of the screen dots on the press, Moire patterns, color shifts, etc.
A lot of variants of the dot size modulation screening have been disclosed in order to remedy these artifacts but none of them was capable of completely eliminating the enlarging of the screen dots on the press and the Moire patterns and dot frequency modulation screening techniques have therefore been suggested to further reduce the problem.
According to frequency modulation screening the distance between the halftone dots is modulated rather then their size. This technique, although well known in the field of low resolution plain paper printers, has only recently obtained much attention for offset printing and other high end printing methods.
Methods for making a lithographic printing plate according to the silver salt diffusion transfer process comprising the steps of frequency modulation screening an original to obtain screened data and image-wise exposing an imaging element according to said screened data, said exposure being scan-wise and/or said imaging element having a flexible support and/or a photosensitive layer comprising a direct positive silver halide emulsion have been disclosed in e.g. EP-A 620673, EP-A 620674 and EP-A 94201942.3.
When a mono-sheet DTR offset printing plate of the second type is prepared comprising the steps of frequency modulation screening an original to obtain screened data and image-wise exposing a precursor of a mono-sheet DTR offset printing plate of the second type according to said screened data and developing a thus obtained image-wise exposed lithographic printing plate precursor by means of an alkaline processing solution in the presence of a developing agent and a silver halide solvent, this printing plate has a constrained tone range in print, a restricted development latitude and a marked loss of the tone value over the whole tone scale but especially in the lower range of said scale in function of the number of printed copies when said imaging elements does not comprise halftone dots of more than 21 .mu.m.